The invention relates to improvements in bobbin cases for sewing machines, and more particularly to improvements in bobbin cases with thread tensioning devices.
It is already known to mount an adjustable thread tensioning device on a wall of a bobbin case, for example, in a manner as disclosed in commonly owned Swiss Pat. No. 435 947 to Gegauf. The thread is caused to pass between the external surface of the cylindrical wall of the bobbin case and an arcuate spring which overlies a portion of the external surface. A screw is provided to permit adjustments of the bias of the spring upon the thread, i.e., the braking action of the spring upon the thread can be varied by rotating the screw so as to cause the spring to store different amounts of energy. The patented bobbin case can ensure highly satisfactory tensioning of the thread if the bias of the spring is properly adjusted and if the configuration of the spring is such that adjustments of its bias actually entail changes in the braking action upon the thread. The making of such springs presents problems when the springs are mass-produced because the radius of curvature of the spring must depart, to a predetermined extent, from the radius of curvature of the external surface of the cylindrical wall of the bobbin case. The making of a spring with a radius of curvature which is slightly smaller than that of the external surface of the cylindrical wall is a prerequisite for establishment of an acceptable tensioning or braking action. Moreover, the configuration of the spring in the axial direction of the cylindrical wall must be constant in order to ensure that the tensioning action will not be increased or reduced if the position of the thread in the axial direction of the cylindrical wall changes (as it normally does when the sewing machine is in use). In other words, it is necessary to ensure that the curvature of the spring in the circumferential direction of the cylindrical wall match or approximate a predetermined value as well as that the configuration of the spring be uniform in the axial direction of the cylindrical wall. This cannot be readily achieved if the spring is to be mass-produced at a reasonable or low cost.
U.S. Pat. No. 3,381,642 to Bono discloses a modified bobbin case for sewing machines wherein the lower thread is tensioned by a thin resilient leaf element which is urged against the external surface of the cylindrical wall of the bobbin case by an arcuate tension regulating ring segment. This bobbin case is also incapable of invariably ensuring that the thread is subjected to a constant tensioning action in spite of repeated changes of position of the thread with reference to the cylindrical wall and the leaf element. The reason is the same as explained above in connection with the proposal of Gegauf, i.e., it is difficult to ensure that the width of the clearance between the external surface of the cylindrical wall and the leaf element will remain uniform in each and every part of the clearance.